Some of New Zealand's historic buildings are about to be hit by a multibillion-dollar bombshell, as the Government forces their strengthening, experts claim.

Marcus Beveridge and Tina Hwang, specialist property lawyers at Queen City Law, say the cost of upgrading around 15,000 to 25,000 heritage structures has been put as high as $100 billion, well exceeding the leaky-buildings crisis.

But Building and Construction Minister Maurice Williamson said charges would be nowhere near the $100 billion mark.

Officials had estimated $1.7 billion under the new proposals, he said, and opponents were suggesting extreme action. That wouldn't be necessary, and not every building would need work, due to exclusions including houses, wool sheds, barns, little-used community halls and perhaps rural churches.

Mr Beveridge and Ms Hwang, who represent many investors and developers, said seismic upgrade of earthquake-prone buildings could cost landlords six times as much as the mid-range estimate for leaky homes.

"The leaky-building saga is estimated to be worth $15 billion and there've been contrary voices on how much the seismic issue is worth," they said. Rotten homes could be fixed in a piecemeal fashion, but weak, dangerous old buildings were much harder to upgrade.

"Leaky buildings had the option of targeted repairs in addition to a full repair. Will a full repair here be required? Would it be economically sustainable? There will be a greater debate over the economics," the lawyers said.

Many tenants were deserting buildings because they were classed as dangerous and insurers were rejecting cover for landlords and tenants.

Chris Darby, Devonport-Takapuna Local Board chairman and Devonport Business Association member, is worried about buildings in his area, saying the ramifications could be far-reaching and tenants were already demanding information.

"Current and potential tenants are increasingly asking agents and landlords about the structural integrity of historic buildings.

"There is cause for concern where the perception is that historic buildings per se are not worth the risk or trouble of buying or leasing."